With increasing development of science and technology, a variety of electronic devices are developed toward minimization, high integration and high power. During operation of an electronic device, a great deal of heat is generated by the electronic components of the electronic device. If the heat fails to be effectively dissipated away, the elevated operating temperature may result in damage, short circuit or deteriorated performance of the electronic device. For effectively removing the heat, a heat-dissipating device is usually installed within or beside the electronic device to exhaust the heat to the surroundings. Moreover, it is critical to increase the efficiency of the heat-dissipating device.
A fan is one of the most common heat-dissipating devices. FIG. 1A is a schematic perspective view illustrating a conventional fan. As shown in FIG. 1A, the fan 1 includes a frame 11, a hub 12 and a plurality of blades 13. The hub 12 is disposed within the frame 11. The blades 13 are arranged around the hub 12 and connected with the hub 12. In addition, a motor (not shown) is installed within hub 12. As the hub 12 is driven to rotate by the motor, the blades 13 arranged around the hub 12 are synchronously rotated to produce airflow to dissipate heat.
The hub 12 and the blades 13 are also collectively referred as an impeller. FIG. 1B is a schematic view illustrating an impeller of the fan of FIG. 1A and the airflow direction, in which there is no back pressure. Please refer to FIGS. 1A and 1B. Since the blade 13 is inclined against the rotating direction of the fan 1, a clearance gap 11a is formed between the tip part 130 of the blade 13 and the inner wall of the frame 11. In a case that there is no back pressure (i.e. free flow), the airflow will be directed from a pressure side 131 of the blade 13 to a suction side 132 of the blade 13 through the clearance gap 11a (i.e. in the direction indicated as the arrow A). In such way, the local pressure may be lost and a pressure fluctuation at the suction side 132 of the blade 13 may occur. Under this circumstance, since the airflow fluctuates upwardly and downwardly in the clearance gap 11a, the wideband or narrowband noise of the fan 1 is increased, the amount of airflow inhaled by the fan 1 is reduced, and the performance of the fan 1 is impaired.
FIG. 1C is a schematic view illustrating an impeller of the fan of FIG. 1A and the airflow direction, in which there is a back pressure. Due to the back pressure, the pressure acting on the pressure side 131 of the blade 13 is increased. That is, the airflow leaks out through the clearance gap 11a more easily. The leaked airflow may result in vortex on the suction side 132 of the blade 13 (in the direction indicated as the arrow B). Under this circumstance, the pressure fluctuation of the flow field on the suction side 132 of the blade 13 becomes more serious, the output static pressure of the fan 1 is reduced, and the efficiency of the fan 1 is impaired.
Generally, as the clearance gap 11a between the fan 11 and the blade 13 is decreased, the possibility of causing turbulent flow is reduced and the efficiency of the fan 1 is increased. However, when the material strength limitation and the deformation extent of the fan 1 are taken into consideration, the size of the clearance gap 11a is positively related to the dimension of the fan 1. That is, the size of the clearance gap 11a fails to be arbitrarily reduced. If no proper measure is taken to reduce the airflow leakage, the output pressure of the fan 1 is reduced. Under this circumstance, the performance is impaired, the efficiency is reduced, and the noise is increased.